The present invention relates to a structure for a wire-dot printing head, and more specifically, to an adjusting mechanism for adjusting a plate spring for the positional adjustment of an armature.
U.S. Pat. No. 4,225,250 (Segmented Ring Magnet Print Head) discloses a wire-dot printing head having an elastic printing hammer which is held at a rest position during a nonprinting period by the magnetic attraction of a permanent magnet and is released for printing operation.
Such a wire-dot printing head is designated generally as a spring-charged printing head, in which the magnetic flux lines of the permanent magnet forms a closed magnetic circuit. This closed magnetic circuit is formed of the permanent magnet, a base yoke, a base, the core of an electromagnet, an armature fixed to a plate spring, an armature yoke, a spacer, and a magnet yoke. The armature is attracted to the core by the agency of the closed magnetic circuit against the resilient force of the plate spring. In a nonprinting state, the plate spring is strained to store strain energy.
In the printing operation, the exciting coil of the electromagnet is energized by a drive signal to generate a magnetic flux of a polarity reverse to that of the closed magnetic circuit formed by the permanent magnet, and thereby the magnetic flux of the permanent magnet is cancelled to release the armature from the core. Then, the strain energy stored in the plate spring drives the armature to project a print wire fixed to the free end of the armature to press the tip of the print wire against a recording sheet for printing.
In such a conventional printing head, it often occurs that the difference between the attraction of the core and the resilient force (strain energy) of the plate spring, namely, armature holding force, varies. When the variation of the armature holding force is particularly large, the armature releasing timing varies between the armatures, and hence the printing period of the dot printing head is caused to vary, and at the worst, it is impossible to achieve the printing operation under a set driving condition, in which the dot printing head cannot be used. Furthermore, if the dot printing head is usable, prints do not come out clearly or come out in different density degrading print quality.
To eliminate such drawbacks, the assignee of the present invention proposed a wire dot-printing head in U.S. Pat. No. 4,692,043 (Ando et al.). This previously proposed wire dot-printing head has a plate spring having a branching portion near the fixed end thereof, and an adjusting screw having a tip in contact with the branching portion and capable of being operated from outside. The distortion of the plate spring is adjusted by operating the adjusting screw.
However, such an arrangement entails problems that additional parts, namely, the adjusting screws, are necessary for adjusting the distortion of the plate springs, that the adjusting screws must be screwed in place one by one in the manufacturing process, and that the distortion of the plate springs must be adjusted to an optimum distortion by operating the corresponding adjusting screws one by one. Such adjusting work must be repeated a number of times equal to the number of the plate springs, for example, twenty-four times when the dot printing head has twenty-four wires, hence twenty-four plate springs, to form a character by a matrix of twenty-four dots.